Two other types of EEG signals that can be used for controlling a BCI are the mu- and beta-rhythms. Beta-rhythm is one of the fastest normal EEG rhythms with a frequency of 13-30Hz. It can occur naturally when awake or during light sleep. The mu-rhythm is essentially a special kind of alpha rhythm (8-12Hz), i.e. what is normally seen in an EEG of a healthy and awake person. Per definition, alpha rhythms occurs in the region of the visual cortex, whereas the mu rhythm originates above the motor cortex, which is the highest instance for all kinds of movement (more).
Theoretically, mu- and beta-rhythms are ideal for controlling a neuroprosthesis as the signals are - due to their origin in the motor cortex - as close as possible to the structures the brain normally uses for movement decisions. Mu-rhythm varies with activity, especially on the side of the brain which lies opposite to the limb movement (more). Furthermore, these variations also occur when movement is only imagined, making mu-based BCIs promising for patients who lost their ability to move only recently.